For capturing an image, a solid-state image capturing element such as a CCD image sensor has been conventionally used. Further, for displaying an image, display devices such as a CRT, a liquid crystal display panel, and a plasma display panel have been conventionally used. When image data (electric signal) converted from an image signal (optical signal) by the solid-state image capturing element is, for example, processed or transmitted, the image data is required to be accumulated in a memory. Therefore, in an image processing as described above, a storage element (memory) for storing an electric signal related to an image is necessary in addition to the solid-state image capturing element.
In order to cope with such a request, there is disclosed a technique to form a volatile memory element constituted by a photodiode, a capacitor element, and a MOS-type transistor, and a non-volatile memory element on a semiconductor substrate. After an optical signal received by the photodiode is converted into an electric signal in the volatile memory element, the electric signal is stored in a non-volatile memory.
However, in order to store the electric signal in the non-volatile memory, it is necessary to apply a voltage to a terminal of the volatile memory element to inject an electric charge as the electric signal stored in the non-volatile memory into the volatile memory element. In other words, when the electric signal is to be stored (written) in the memory, an external voltage is required to be applied. Not only a writing operation is complicated, but also an external electric source is additionally necessary for the purpose of writing.
Further, there is disclosed a technique in which a photodiode and a non-volatile memory are formed on a semiconductor substrate and an electron is injected into a floating gate of the non-volatile memory in advance and a negative voltage is applied to a control gate of the non-volatile memory. By supplying a positive electric charge originated in an electric signal converted from an optical signal in the photodiode to pull the electron out of the floating gate, the electric signal is stored in a non-volatile memory element.
However, also in this case, in order to store (write) the electric signal in the non-volatile memory, a negative voltage is required to be applied to a control gate of the non-volatile memory. Not only a writing operation is complicated, but also an external electric source is additionally necessary for the purpose of writing.